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This 13V 2.7A TOP267 design recently failed the radiated and conducted emission but passed all the other EMC tests (immunity, harmonics and flicker, transient burst etc.) The circuit had a RCZD clamp and the drain voltage waveform was squarish. Circuit and waveform are attached.

Hence I changed to use the 'lossless snubber circuit' according to Abraham Pressman's book. It's a CLD circuit. We haven't gone back for another EMC test yet but the waveform now looks much rounder. Circuit and waveform are attached. We also have to add a series resistor on the HV side of the optocoupler due to safety standard requirement. With these changes, we made another board with different layout so to fit everything in. And one obvious problem we have now is that the PS has problem starting up sometimes and it has to try for a few seconds and eventually recover. How can I fix this problem?

Please also check my layout and suggest improvement as needed for emc purpose.

Many thanks.

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PI forum 120712.pdf2.72 MB